Method and apparatus for nuclear magnetic resonance measuring while drilling

ABSTRACT

A method for manufacturing NMR measurement-while-drilling tool having the mechanical strength and measurement sensitivity to perform NMR measurements of an earth formation while drilling a borehole, and a method and apparatus for monitoring the motion of the measuring tool in order to take this motion into account when processing NMR signals from the borehole. The tool has a permanent magnet with a magnetic field direction substantially perpendicular to the axis of the borehole, a steel collar of a non-magnetic material surrounding the magnet, antenna positioned outside the collar, and a soft magnetic material positioned in a predetermined relationship with the collar and the magnet that helps to shape the magnetic field of the tool. Due to the non-magnetic collar, the tool can withstand the extreme conditions in the borehole environment while the borehole is being drilled. Motion management apparatus and method are employed to identify time periods when the NMR measurements can be taken without the accuracy of the measurement being affected by the motion of the tool or its spatial orientation.

This application is a continuation of application Ser. No. 10/108,182filed Mar. 27, 2002 now U.S. Pat. No. 6,583,621 which is a continuationof application Ser. No. 09/919,129, filed Jul. 31, 2001, which is acontinuation of application Ser. No. 09/232,072, filed Jan. 15, 1999,now U.S. Pat. No. 6,268,726, which claims priority of provisionalapplications Ser. No. 60/071,612 and Ser. No. 60/071,699, both filedJan. 16, 1998. The content of the above applications is incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates generally to a method and apparatus for makingpulsed nuclear magnetic resonance (NMR) measurements of earth formationswhile drilling a borehole. More specifically, the invention is directedto a NMR measurement-while-drilling (MWD) tool having the requiredmechanical strength and measurement sensitivity, and a method andapparatus for monitoring the motion of the measuring tool in order totake this motion into account when processing NMR signals from theformation surrounding the borehole.

BACKGROUND AND SUMMARY OF THE INVENTION

Various methods exist for performing downhole measurements ofpetrophysical parameters in geologic formations. Pulsed NMR logging isamong the most important of these methods, and was developed primarilyfor determining parameters such as formation porosity, fluidcomposition, the quantity of movable fluid, permeability, and others.Importantly, NMR measurements are environmentally safe and areunaffected by variations in the matrix mineralogy.

In a typical NMR measurement, a logging tool (measurement device) islowered into a drilled borehole to measure properties of the geologicformation near the tool. Then, the tool is extracted at a known ratewhile continuously taking and recording measurements. At the end of theexperiment, a log is generated showing the properties of the geologicformation along the length of the borehole. This invention relatesprimarily to an alternative measurement, in which pulsed NMR logging canbe done while the borehole is being drilled. The advantages of thelatter approach in terms of saving both time and costs are apparent.Yet, very little has been done so far in terms of developing practicalNMR logging-while-drilling (LWD) or measurements-while-drilling (MWD)solutions. Two of the main stumbling blocks appear to be the verystringent requirements concerning the mechanical strength of the device,as well as problems associated with inaccuracies of the received signalsdue to motions of the tool. The present invention addresses successfullyboth issues and therefore is believed to make a significant contributionover the prior art.

In order to more fully appreciate the issues discussed in detail next, abrief overview of NMR methods for measuring characteristics offormations surrounding a borehole is presented first. The interestedreader is directed, for example, to the following article: Bill Kenyonet al., Nuclear Magnetic Resonance Imaging—Technology for the 21stCentury, OILFIELD REV., Autumn 1995, at 19, for a more comprehensivereview. The Kenyon article is incorporated herein by reference.

Basically, in the field of NMR measurements of earth formationssurrounding a borehole, a downhole static magnetic field is used toalign the magnetic moment of spinning hydrogen (H) protons in theformation in a first direction, the direction of the static magneticfield. In order to establish thermal equilibrium, the hydrogen protonsmust be exposed to the polarizing field for a multiple of thecharacteristic relaxation time, T₁. Then, the magnetic component of aradio frequency (RF) electromagnetic wave pulse, which is polarized in asecond direction orthogonal to the static field, is used to tip theprotons to align them in a third direction that is orthogonal to boththe first and the second direction. This initial RF pulse is thus calleda 90° pulse. Following the 90° pulse the protons in the formation beginto precess about the axis of the first direction. As a result, theprotons produce an oscillating magnetic field. If an antenna is placedinto the oscillating magnetic field, the oscillating magnetic field willproduce an oscillating electric current in the antenna. Because theamplitude of the induced electrical signal is proportional to theporosity of the earth formation being measured, the signal may becalibrated to measure formation porosity. However, due to dephasing andirreversible molecular processes, the induced signal decays rapidlyafter the RF pulse is removed. Consequently, when the antenna is usedboth to transmit the RF pulses and to receive the induced NMR signal asin one embodiment of this invention, this first NMR signal may not beobservable because the antenna electronics are still saturated fromresidual effects of the 90° RF pulse. Therefore, the NMR signal must berebuilt as a spin echo, as discussed below, so that it may be measured.

Additionally, the behavioral characteristics of the protons afterremoval of the RF pulse can be used to garner information about otherformation properties, such as pore size distribution and permeability.Immediately after the 90° RF pulse is turned off, the protons precess inphase. However, due to inhomogeneities in the static magnetic field andirreversible molecular processes, the protons begin to dephase, whichcauses the induced signal to decay. Nevertheless, the dephasing due toinhomogeneities in the static magnetic field is partially reversible.Therefore, by applying a 180° RF pulse, the instantaneous phases arereversed such that the protons gradually come back into phase, thusrebuilding the induced signal. The antenna can detect this signalbecause the time required for rebuilding the signal is long enough toallow the antenna electronics to recover from the 180° RF pulse. Afterthe signal peaks at the time when the protons are back in phase, thesignal will then begin to decay due to dephasing in the oppositedirection. Thus, another 180° RF pulse is needed to again reverse theinstantaneous phases and thereby rebuild the signal. By repeating aseries of 180° RF pulses, the signal is periodically rebuilt after eachdephasing, although each rebuilding is to a slightly lesser peakamplitude due to the irreversible molecular processes. Eventually, theirreversible processes prevail such that no further rephasing ispossible and the signal dies out completely. Each rebuilding of thesignal in this manner is called a spin echo, and the time constantassociated with the decay of the spin echo amplitudes is called thetransverse relaxation time, T₂.

Because experiments have shown that T₂ is proportional to the pore sizeof the formation, calibration and decomposition of T₂ yields a measureof the formation's pore size distribution. Moreover, when combined withthe porosity measurement, T₂ yields an estimate of the formation'spermeability. As noted above, the NMR signal may also be calibrated toobtain other formation characteristics, such as free fluid volume, boundfluid volume, fluid identification, and diffusion coefficients. Becausethe drilling mode of operation using a preferred embodiment of thisinvention may allow for enough time to develop only one spin echo, or atmost a few spin echoes, the apparatus may achieve only porosity andlimited T₂ measurements while drilling. However, the other types of NMRmeasurements discussed above are possible in nondrilling modes ofoperation, such as stationary tool, sliding or wiping tool.

From the preceding discussion it should be apparent that in order toenable accurate NMR measurements it is important that the same protonsbe tipped and rephased for each successive spin echo. Excessive movementof the tool in the borehole during NMR measurement can destroy theaccuracy of the measurement by changing the location of the measurementvolume, i.e., which protons in the formation are affected by theinteraction of the static and RF pulse magnetic fields. Therefore, ifthe motion of the tool during NMR measurement is not known, whichgenerally is the case in a logging while drilling environment, the NMRmeasurement may not be reliable.

The present inventors know of three issued patents directed to practicalNMR measurements while drilling: U.S. Pat. No. 5,705,927 issued Jan. 6,1998, to Sezginer et al.; U.S. Pat. No. 5,557,201 issued, Sep. 17, 1996,to Kleinberg et al.; and U.S. Pat. No. 5,280,243 issued Jan. 18, 1994,to Miller. Of these references, the '201 patent more specifically showshow to improve the tool's susceptibility to lateral tool motion byincreasing the radial dimension of the measurement volume. However, theaxial length of the sensitive zone (i.e., the measurement volume) of the'201 patent is on the order of two to four inches, whereas that of thepresent invention is on the order of two feet. Thus, the susceptibilityof the present invention to axial tool movement is greatly improved overthe prior art. Indeed, for typical drilling rates, axial movement of thepresent NMR tool has a negligible impact on the quality of the NMRmeasurement. Importantly, none of the three patents recited abovediscloses any means to monitor the tool motion to assure a drillingoperator that the NMR measurements are accurate.

One method of dealing with the motion of the NMR tool in accordance withthe present invention is to monitor the tool motion during NMRmeasurement and discard the measurement if the tool motion is abovemaximum acceptable limits. For example, in a preferred embodiment ofthis invention, testing has shown that the lateral velocity of the toolmust be less than or equal to about 0.2 m/s to preserve the integrity ofthe NMR measurement. Accordingly, an important aspect of the presentinvention is the disclosure of a method for monitoring the tool motionby using two pairs of accelerometers located at the ends of twocoplanar, orthogonal drill collar diameters. The accelerometers are usedto measure the lateral acceleration of the tool, and the acceleration isintegrated once to obtain the velocity and twice to obtain thedisplacement.

Although U.S. Pat. No. 4,958,125, issued Sep. 18, 1990, to Jardine etal., discloses a similar method and apparatus for determiningcharacteristics of the movement of a rotating drill string, the methodfor determining lateral acceleration in the '125 patent is directed to avertical drill string orientation. Referring to FIG. 8, fouraccelerometers oriented such that ac1 and ac2 are on one axis and ac3and ac4 are on an orthogonal axis, both orthogonal to the general axisof rotation, the '125 patent sets forth the following equations ofmotion:

ac 1=ac+ax cos d

ac 2=ac−ax cos d

ac 3=ac+ax sin d

ac 4=ac−ax sin d  Eqs. [1]

where ac is the centripetal acceleration, ax is the lateralacceleration, and d is the angle between the ac1/ac2 axis and thelateral acceleration vector. From Eqs. [1], the '125 patent derives thefollowing expressions for the rotation speed, S, and lateralacceleration, ax:

S=[60/2π]*[(ac 1+ac 2)/(2r)]^(1/2)  Eq. [2]

ax={[(ac 1−ac 2)/(2)]²+[(ac 3−ac 4)/(2)]²}^(1/2)  Eq. [3]

The direction of the lateral acceleration is determined by the followingexpression:

tan d=(ac 3−ac 4)/(ac 1−ac 2)  Eq. [4]

However, Eqs. [1] do not contain any gravitational acceleration terms.Thus, Eqs. [1] correctly describe the tool motion only if the tool isoriented vertically such that the lateral component of the gravitationalacceleration is zero.

To describe the tool motion accurately if the tool is in some general,inclined orientation, the equations of motion must include thegravitational acceleration terms as follows:

ac 1=ac+ax cos d+g sin α cos e

ac 2=ac−ax cos d−g sin α cos e

ac 3=ac+ax sin d−g sin α sin e

ac 4=ac−ax sin d+g sin α sin e  Eqs. [5]

where g is the earth's gravitational constant (9.81 m/s²), α is theinclination angle of the tool axis with respect to the vertical (asshown in FIG. 7), and e is the angle between the a1/a2 axis and the gsin α direction (as shown in FIG. 8). As noted in the '125 patent, Eq.[2] still holds true for a general orientation because of fortuitousplus and minus signs on the gravitational acceleration terms. However,neither Eq. [3] nor Eq. [4] holds true for a general orientation. Thus,for a general orientation, another method is needed to determine themagnitude and direction of the lateral acceleration, ax.

In a specific embodiment, the present invention solves this complicationcaused by the presence of the gravitational acceleration terms in ageneral, inclined drill string orientation by incorporating a high-passfilter to eliminate those terms. This solution is possible because therotational frequencies of typical drilling speeds are well below thefrequencies of the lateral accelerations of interest. Thus, thegravitational acceleration terms, which vary periodically at thefrequency of the drill string rotation speed, can be safely eliminatedwithout corrupting the lateral acceleration signals. After filtering inthis manner, the governing equations of motion revert back to Eqs. [1],and Eqs. [3] and [4] may be used to determine the magnitude anddirection of the lateral acceleration. Then, the lateral velocity andlateral displacement may be obtained by integrating the lateralacceleration once and twice, respectively. By comparing the measuredmotion to the allowable motion criterion, it is possible to modify theNMR measurement to optimally suit a given drilling environment.

Based on information from the motion sensors and based on parameters setby the operator before the tool was deployed, in accordance with thepresent invention the tool enters one of the following operating modes:

(a) Shutdown. This mode is selected anytime the tool detects thepresence of metallic casing and/or is on the surface, or detects motionphenomena that make NMR measurements impossible.

(b) Wireline emulation. When no motion is detected, the tool attempts toemulate NMR measurements as typically done by wireline NMR tools.

(c) Normal drilling. During normal drilling conditions, moderate lateralmotion is present, which allows for abbreviated NMR measurements.

(d) Whirling. During whirling, lateral motion is violent, but short timewindows exist during which the lateral velocity drops to a point, wherea porosity-only measurement is possible. The tool identifies thesewindows and synchronizes the NMR measurement appropriately.

(e) Stick-slip. In this drilling mode, windows exist in which short NMRmeasurements are possible, interspersed with periods of very highlateral/rotational motion. Again, the tool identifies these windows andsynchronizes the NMR measurement appropriately.

The motion management aspect of this invention provides an algorithm topredict desirable time windows in which to make valid NMR measurements.As seen in FIG. 9, experiments have identified three distinct types oftool motion: (1) normal drilling; (2) whirling; and (3) stick-slip.These three types of motion are identifiable based on the time historiesof the rotation speed, lateral velocity, and lateral displacement of thetool. In normal drilling motion, the lateral velocity of the tool istypically within acceptable limits so that valid NMR measurements may bemade at almost any time. Sample plots of typical tool motion duringnormal drilling are shown in FIGS. 10A(1)-(6) and 10B(1)-(6). Inwhirling motion, the lateral velocities are usually outside acceptablelimits, which makes valid NMR measurements difficult to obtain. However,as shown in FIG. 9, by monitoring the velocity, acceleration, and timeduration in which the velocity remains within certain prescribed limits,it is possible to predict acceptable NMR measurement periods duringwhirling. Sample plots of typical tool motion during whirling are shownin FIGS. 13A(1)-(6) and 13B(1)-(6). In stick-slip motion, the drill bitperiodically sticks to the borehole wall and then slips away, causingthe drill string to periodically torque up and then spin free. Sampleplots of typical tool motion during stick-slip are shown in FIGS.11A(1)-(6); 11B(1)-(6); 12A(1)-(6) and 12B(1)-(6). During the stickphase, the tool is virtually stationary thus providing a good timewindow in which to make NMR measurements. In contrast, the lateralvelocity during the slip phase may be outside acceptable limits,depending on such variables as bit type, formation strength, andstiffness and length of the drill string. By analyzing the timehistories in this manner, acceptable time windows may be predicted inwhich to make valid NMR measurements.

The motion identification aspect of this invention also provides anotherbenefit with regard to drilling tool damage reduction and service lifeenhancement. Because whirling and stick-slip motion can be damaging todrilling tools, this invention's capability of identifying these typesof motion is very useful to a drilling operator. Specifically, if theoperator knows that the drill collar is undergoing whirling orstick-slip motion, the operator can make appropriate changes to theweight-on-bit and rotation speed parameters and thereby change themotion to approach normal drilling as much as possible. As a result,tool damage is reduced and service life is enhanced.

Alternatively, this invention also incorporates a method for measurementof the tool motion by means of acoustic sensors. In this alternative, atleast two acoustic sensors are placed on the perimeter of the drillcollar. If only two acoustic sensors are used, they are preferablyplaced on orthogonal diameters (i.e., spaced 90° apart). These acousticsensors detect the distance from the tool to the borehole wall and thusdirectly measure the lateral displacement of the tool in the borehole.Because the sampling rate of the acoustic sensors is much greater thantypical drill string rotation speeds, the rotation is negligible withrespect to the displacement calculation. In turn, the displacement maybe differentiated once to obtain the velocity and twice to obtain theacceleration. Again, by comparing the measured tool motion to theallowable motion criterion, the corresponding NMR measurement may beappropriately retained or discarded. This quality control check may beaccomplished using either displacement or velocity data, because thedisplacement criterion is over a known time span (i.e., the time betweenthe 90° pulse and the signal acquisition window). Alternatively, whencombined with an inclinometer or magnetometer measurement from which thedrill collar rotation speed may be obtained (as described in U.S. Pat.No. 4,665,511 issued May 12, 1987, to Rodney and Birchak), the timehistories of the motion may be used in conjunction with the predictionalgorithm discussed above to predict acceptable NMR measurement windows.The same approach could also be used with contact sensors (as describedin U.S. Pat. No. 5,501,285 issued Mar. 26, 1996, to Lamine andLangeveld) instead of acoustic sensors to measure the tool displacementby measuring the change in resistance through the drilling mud.

Another alternative for the motion management aspect of this inventionis to correct the NMR measurement for losses in the NMR signal due tolateral motion during measurement. That is, by measuring the lateraldisplacement of the tool during NMR measurement as described above, thechange in the sensitive volume can be calculated, which in turn allowsthe calculation of an appropriate correction factor to be applied to thereceived NMR signal to compensate for tool movement. This correction maybe accomplished by using displacement information derived from any ofthe three sensor types discussed above (accelerometers, acousticsensors, or contact sensors). An advantageous embodiment of the acousticsensor form of this invention meets a tool position error specificationof about 5%, which allows correction of the NMR signal to within about95% of the signal for a stationary tool. Therefore, rather than discardan NMR measurement taken during a period of what would otherwise beexcessive lateral motion, the NMR signal can be corrected to compensatefor the tool motion.

Another aspect of the motion-detection problem of this invention is theoperation of phase-alternated signal averaging, which is typical for NMRdata acquisition and which is further described below. The salientfeature of phase alternation is the coherent accumulation of NMR data,coupled with the progressive suppression of non-NMR artifacts. A largecontribution to the latter comes from magneto-acoustic oscillations(“ringing”) within the ferrous material as well as pulse-inducedvibrations in current-carrying conductors (also customarily termed“ringing”). The pair-wise subtraction process relies on the fact thatthese artifacts are more or less repeatable, given the same excitationthrough a series of 180° RF pulses. It has been determined byexperimentation that the patterns of these artifacts tend to changecyclically with the tool's orientation and bending.

This invention provides for a means to accommodate ringing patternchanges and the effects of partial de-coupling of the magnet and antennaby taking advantage of the fact that the generally cyclical andrepetitive nature of the drilling process is the source of the dynamicgeometry changes. Practically all drilling involves rotation of a drillbit. Rotation is provided by bulk rotation of the entire drill string,use of a mud motor, or by a combination of both. As an example, the bitcenter orbit plots 10B3, 11B3, 12B3 and 13B3 show that the position ofthe bit is very repetitive and essentially duplicated with eachrevolution for drilling modes other than whirling. By measuring one ofthe many manifestations of this rotation, including oscillations of theon-rotating section, the NMR measurement can be repeatedly synchronizedto a particular and repeatable geometry.

Magnetic and gravity tool face angles are among the most available meansfor tracking tool orientation on a rotating or oscillating drill string.Bending stress, position derived from integration of acceleration data,sonic sensors or contact sensors are other examples of sensors suitablefor synchronization of the NMR measurement.

In an important aspect, this invention also provides a permanent magnet,preferably having tubular construction, which produces a static magneticfield that is oriented in a substantially orthogonal direction of boththe axis of the borehole and the drilling device, and that diminishes inmagnitude by about the square of the distance from the magnet. Over therelatively thin sensitive volume (1.5 mm thickness) the radial fieldgradient is essentially constant.

As shown in FIG. 6B, the magnetic field is that of a linear dipole witha field direction that depends on the azimuthal direction of the tool.Although some prior NMR tools have used magnets having a linear dipolemagnetic field (such as that disclosed in U.S. Pat. No. 4,710,713),those tools did not comprise a tubular magnet through which drilling mudmay be pumped nor are these magnets meant to be rotated. Additionally,although the Kleinberg et al. '201 patent discussed above comprisestubular magnets, the magnetic field produced is not a linear dipole. Thetubular construction used in accordance with the present inventionprovides a central cavity through which drilling mud may be pumped toenable NMR measurement while drilling. Also, the magnetic field producedby the magnet has an essentially constant gradient within themeasurement volume which, when combined with an RF pulse that is tunedto the proper frequency, produces a more uniform annular sensitiveregion (measurement volume) that is completely within the earthformation.

Another aspect of the present invention is the use of a magnet that iscomprised of multiple segments. Thus, in accordance with the presentinvention it is possible to tailor the resultant field by tuning thecontributions from the individual segments. Such tuning may beaccomplished by (a) selective demagnetization, which is possible for theSmCo5 variant of samarium-cobalt material, by (b) adjusting the volumefor each segment, or (c) by adjusting the polarization direction of eachindividual segment. Such freedom in field shaping is advantageous if itis necessary, as in the present invention, to pre-compensate themagnetic field in order to accommodate the effects of soft-magneticmaterial in the vicinity of the magnet.

It may not be obvious even to a person skilled in the art that themagnetic field from a linear dipole is suitable for measurements whiledrilling, where the magnet is rotated with respect to the formation atrates up to about 300 RPM. Any given point within the sensitive volume36 in FIG. 6B experiences a magnetic field of approximately constantmagnitude, but that rotates synchronously with the drill collar. It mayappear that for hydrogen nuclei with longitudinal relaxation times T₁ ofseveral seconds, these spins would not align themselves properly with arotating field that completes a rotation in much less time than it takesfor polarization. However, the relaxation time T₁ only governs thebuild-up of the magnitude of the nuclear polarization. A change indirection can be followed much faster and depends on the resonant Larmorfrequency of the nucleus. At a typical field strength of 117 gauss, theresonance frequency is 500 kHz. The condition for Adiabatic Fast Passage(AFP), under which the nuclear polarization follows a change indirection of the polarizing field virtually instantaneously, is that therate of change must be much less than the period of the Larmorfrequency. Comparing 500 kHz with a maximum rotational frequency of5/sec, we find a ratio of 100,000:1, which satisfies the requirement foran AFP condition by a wide margin. Consequently, although using arotating drill collar which causes all fields within the formation toconstantly change direction, the hydrogen nuclei always follow thechanging directions without noticeable delay.

As is known in the art of NMR measurement, the frequency of the RF pulsemust be tuned to the Lamior frequency (f_(L)) (in Hertz) of the hydrogenprotons, which is given by the following equation:

f _(L)=4258B ₀  Eq. [6]

where B₀ is the magnitude of the static magnetic field (in Gauss).Because the static magnetic field of this invention decreasesmonotonically as a function of radial distance from the tool, thelocation of the sensitive zone may be selected by choosing a value of B⁰that coincides with the desired radial distance from the tool. In thismanner, the sensitive zone can be fixed entirely within the earthformation to be measured, instead of partially in the borehole. Asdescribed in U.S. Pat. No. 4,350,955, issued Sep. 21, 1982, to Jacksonand Cooper, if the sensitive zone is partially in the borehole, thatsituation presents a serious drawback to the system in that the NMRsignal from the borehole fluid overwhelms the signal from the earthformation.

Prior art systems have attempted to solve this problem by doping theborehole fluid with chemicals (as described in the '955 patent), whichwas time consuming and expensive, or by utilizing a gradient coil toproduce an additional pulsed magnetic field to cancel the boreholesignal (as described in the '201 patent), which further complicated thesystem. Therefore, the capability of the present invention to fix thesensitive zone completely within the earth formation without anyadditional apparatus constitutes a valuable improvement over most of theprior art. Moreover, the static magnetic field of this invention as afunction of radial distance from the tool is such that the location ofthe Larmor frequency for the sodium (Na) quadruple moment lies insidethe tool volume, as shown in FIG. 23. The gyromagnetic ratio of sodiumis 1127 Hertz/gauss. To resonate at the same Larmor frequency, sodiumrequires approximately four times the field strength, a condition thatis met at a diameter of about one-half of the sensitive diameter ofhydrogen. The hydrogen diameter of 13.5 inches has been chosen tocontain the potential sensitive volume for sodium with a diameter of6.75 inches entirely within the tool. Therefore, this invention has theadded advantage that it is not sensitive to sodium in the boreholefluid. It should be obvious to someone skilled in the art to scale theresonance diameters appropriately for tools used in different-sizedboreholes.

The above-described advantage with respect to thecompletely-in-formation sensitive zone is made possible by combining theconstant gradient tubular magnet with a nonmagnetic metal drill collar,an axially elongated antenna, high electrical conductivity shielding forthe antenna, and a ferrite buffer. Although other practitioners in theart were of the opinion that this invention would not work with a metaldrill collar, the inventors have demonstrated the contrary. A metaldrill collar is desirable to increase the tool's strength and durabilityin the harsh downhole environment of high temperatures, pressures, andabrasive fluids and particles and corrosive fluids. The antenna of thisinvention, which is used both to transmit the RF pulses and to receivethe NMR signals, is located on the outside of the drill collar and,along with the magnet, has a relatively long axial dimension to producea sensitive volume having a large axial dimension, as described above.In one embodiment, a shield made of high conductivity material (such ascopper) is placed between the outer surface of the metal drill collarand the antenna to reduce acoustic ringing of the metal drill collar dueto the RF pulses and to increase the efficiency of the antenna duringthe transmission of the RF pulses. The shield is thin enough toattenuate surface acoustic waves in the shield, yet it is fixed to thecollar firmly enough to prevent bulk vibration of the shield. Further,the shield is acoustically isolated from the collar to prevent surfaceacoustic waves in the shield. These acoustic and vibrationcharacteristics are enabled by bonding the shield to the collar with athin layer of material having the desired acoustical properties, such asrubber or lead-filled epoxy. In another embodiment, the outer surface ofthe drill is made highly conductive, so that there is no need for aseparate shield.

Further, in accordance with this invention, a layer of ferrite materialis placed between the antenna and the shielding to direct the pulsed RFmagnetic fields into the formation and to further increase theefficiency of the antenna so that it requires less power in thetransmission mode and has increased sensitivity in the receive mode.Preferably, the ferrite material is layered such that a more or lesscontinuous path through the ferrite material exists along the magneticfield lines of the RF field, but repeated discontinuities are introducedin the transversal direction to the RF magnetic field lines. Thesefeatures in combination enable the placement of the sensitive zone farenough away from the tool to be completely in the formation yet stillallow accurate sensing of the NMR signal by the antenna.

Yet another aspect of this invention involves a high-current,low-impedance feed through connector to connect the antenna to theantenna's tuning capacitors. As in the prior art, tuning capacitors areutilized in the antenna electronics (driving circuitry) to match theimpedance of the antenna so that the antenna will resonate at thedesired frequency. However, the capacitors are sensitive items andrequire protection from the high pressures and temperatures of theborehole environment. Before the invention described in U.S. Pat. No.5,557,201, this problem was solved by selecting capacitors with minimalpressure and temperature sensitivities and isolating the capacitors fromthe borehole fluids in an oil-filled compartment of the drill collar.The compartment seal separated the compartment from the borehole fluids,but the seal did not form a pressure seal and therefore the compartmentsaw the ambient borehole pressure. Consequently, the compartment wasfilled with oil to transmit the ambient pressure uniformly around thecapacitors and thereby prevent them from being crushed by the highdifferential pressure. Moreover, because the oil expands and contractswith changing temperature and pressure, these prior art devices had toinclude a means of varying the volume of the compartment to compensatefor the temperature and pressure changes. Thus, such a scheme was verycumbersome.

The '201 invention solved this problem by housing the antenna drivingcircuitry in a compartment that is not only sealed off from the boreholefluids but is also sealed off at constant atmospheric pressure. Thus,the compartment is simply filled with air instead of oil, and there isno need for a volume-regulation device. This method of protecting thecapacitors makes the manufacturing of the tool much simpler and lesscostly. However, because the pressure in the vicinity of the antenna ismuch higher than the pressure in the capacitor compartment, theapparatus for feeding the antenna into the capacitor compartment mustwithstand a severe pressure differential. With such a high pressuredifferential, one would desire to minimize the area of the feed-throughapparatus to minimize the force acting on it. On the other hand, becausethis NMR measurement-while-drilling (MWD) tool requires a very highcurrent in the antenna, the area of the feed-through apparatus must belarge enough to accommodate a conductor of sufficient size to meet thehigh current requirement. Additionally, the feed-through area must belarge enough to supply a sufficient gap between the two antenna wires aswell as to any surrounding metallic material.

This invention solves the problem posed by these conflicting arearequirements by providing a conductor with a corrugated-shapecross-section for the feed-through connector. The corrugated shape ofthe conductor provides sufficient size to carry the high current, yetthe conductor requires much less feed-through area for the connectorthan that which would be required for a conventional, flat cross-sectionconductor. Thus, this corrugated design minimizes the force on thefeed-through connector while still accommodating the necessary current.Moreover, the corrugated design improves the bond between the conductorand the surrounding connector material by providing more bonding area.The connector maintains a stripline interface, thereby minimizing straymagnetic fields and electromagnetic losses.

Yet another aspect of this invention involves a method of mounting theelectronics in the outer portion of the drill collar in such a way as tominimize the drill collar stresses that are transferred to theelectronics. In the borehole, the drill collar is frequently subjectedto bending stresses, axial stresses, and torsional stresses. Thus, whileone side is in compression, the other side is in tension, and thehighest stresses are in the outer portion of the drill collar. Becausethe drill collar rotates during drilling, the drill collar undergoesmany cycles of tensile and compressive stresses. Therefore, anystructure that is fixedly mounted to the drill collar will be subjectedto similar strains as the collar material undergoes at the mountingsurface, and the strains will be highest in the outer portion of thedrill collar. Hence, the outer portion of the collar would seem to be anundesirable location to mount the delicate electronics.

This invention allows the installation of the electronics in the outerportion of the drill collar by means of a mounting scheme wherein thestresses of the drill collar are not appreciably transferred to theelectronics. Specifically, one of the two ends of the electronics moduleis fixedly mounted to the drill collar, but the second end is mounted tothe collar with a sliding connection. Thus, as the drill collar bends,elongates, and otherwise develops stresses, the second end of theelectronics module slides relative to the drill collar and therefore theelectronics module remains relatively free of the drill collar stresses.By virtually eliminating the transferred stresses, this invention allowsthe installation of the electronics in the outer portion of the drillcollar and greatly reduces the potential strain and fatigue problems,such as broken printed circuit board traces and broken leads oncomponents due to over-stress.

Finally, the NMR MWD tool embodying this invention requires very highdownhole power within a very short time period. Specifically, itrequires about 1.5 kilowatts during the short duration of an NMRmeasurement, but only a few watts between measurements For example, atypical load change would be 1 kW for 10 mscc, 50 W for three seconds, 1kW for 10 msec, and so on. Before this invention, existing downholepower generators were not directed to meeting these kind of fluctuatingpower requirements. Therefore, the apparatus embodying this inventionincludes a high power generator to meet the need.

Accordingly, it is an object of this invention to provide an improvedapparatus and method for performing a full range of NMR measurements onformations surrounding the borehole in particular while drilling thataddress the above issues and overcome deficiencies associated with theprior art.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may best be understood by reference to the followingdrawings, in which like reference numerals designate like elements:

FIG. 1 is a schematic axial cross-sectional view of an apparatusconstituting a preferred embodiment of the apparatus of this invention.

FIGS. 2, 3, 4 and 4B are diametrical cross-sectional views of theapparatus shown in FIG. 1 respectively on planes 2—2, 3—3 and 4—4.

FIGS. 4A, 4C and 4D illustrate bonding layers used in accordance withthe present invention.

FIG. 5 is a schematic perspective view showing the configuration of theantenna for the apparatus illustrated in FIG. 1.

FIG. 6A is a schematic sectional view showing the sensitive volumeproduced by the apparatus of FIG. 1.

FIG. 6B is a schematic sectional view similar to FIG. 6A but showing themagnetic flux lines produced by the apparatus of FIG. 1.

FIG. 7 is a schematic elevational view showing the gravitationalacceleration of the apparatus of FIG. 1 when said apparatus is inclinedwith respect to the vertical.

FIG. 8 is a schematic cross-sectional view showing a preferredembodiment for the arrangement of the accelerometers in the apparatus ofFIG. 1.

FIG. 9 is an algorithm block-diagram of the motion management methodused in accordance with a preferred embodiment of the present invention.

FIGS. 10A(1)-(6), 10B(1)-(6), 11A(1)-(6), 11B(1)-(6), 12A(1)-(6),12B(1)-(6), 13A(1)-(6) and 13B(1)-(6) show plots of the lateral motionof the apparatus of FIG. 1 in different motion regimes.

FIG. 14 shows various stages of the change in sensitive volume for theapparatus of the present invention, which are due to lateral motion.

FIGS. 15 and 16 show how the change in sensitive volume may becalculated in accordance with a preferred embodiment of the presentinvention.

FIG. 17 shows a plot of the sensitive volume as a function of lateraldisplacement.

FIGS. 18 and 18A represent schematic cross-sectional views showingalternate embodiments of feed-through connectors.

FIG. 19 is an enlarged axial cross-sectional view of a portion of theapparatus of FIG. 1 showing the magnet assembly and feed-throughconnector.

FIG. 20 is a schematic elevational view of an electronics compartment.

FIG. 21 is a schematic sectional view showing the assembly of thereduced-stress electronics mounting apparatus in the drill collar.

FIG. 22 contains two schematic cross-sectional views of the permanentmagnet magnetized uniformly and nonuniformly.

FIG. 23 is a graph showing the static magnetic field versus radialdistance from the tool for the apparatus of the present invention.

FIG. 24 is a schematic axial cross-sectional view of the power generatorused in a preferred embodiment of the present invention.

FIG. 25 is a schematic cross-sectional view of the fixed and movablemagnets of the power generator.

FIG. 26 is a schematic cross-sectional view taken on the plane F26—F26as shown in FIG. 24.

FIG. 27 is a schematic elevational view of the stop pin and cooperatingstructure.

FIGS. 28A and 28B are schematic views showing the relative positions ofthe fixed and movable magnets of the power generator.

FIGS. 29 through 32 are schematic axial cross-sectional views of anapparatus constituting an alternative advantageous embodiment of thisinvention.

FIG. 33 is a block diagram of the tool electronics.

FIGS. 33(a) and 33(b) are timing diagrams for two versions ofCarr-Purcell-Meiboom-Gill pulse-echo sequences used for NMR measurementsin accordance with a preferred embodiment of this invention.

FIG. 34 is a timing diagram for a single-echo CPMG sequenceincorporating phase alternation.

FIG. 35 is a schematical sketch of the echo amplitudes obtained from aphase-alternated pair, in a stationary tool (a), and during lateralmotion (b).

FIG. 36 shows the NMR data acquisition algorithm used by the signalprocessor of FIG. 33.

FIG. 37 is a schematic cross-sectional view depicting two positions ofan apparatus shown in any of FIGS. 29 through 32.

FIG. 38 is a graph showing the sinusoidal variation of the electricalsignal generated by either the magnetometer or inclinometer, asappropriate, in an apparatus shown in any of FIGS. 29 through 32.

FIG. 39 is a schematic cross-sectional diagram showing a rotatingcoordinate system (x′,y′) and a fixed coordinate system (x,y) for theapparatus of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of the preferred embodiments Section Afocuses on the components of the NMR measurement tool in accordance witha preferred embodiment of this invention, Section B provides a briefdescription of the preferred embodiment for the NMR measurement methodof the present invention using motion management to compensate for toolmotions during NMR logging operations.

(A) The NMR Tool

There are two versions of modern pulse-NMR logging tools in use today:the centralized MRIL® tool made by NUMAR Corporation, and the side-wallCMR tool made by Schlumberger. The MRIL® tool is described, for example,in U.S. Pat. 4,710,713 to Taicher et al. and in various otherpublications including: “Spin Echo Magnetic Resonance Logging: Porosityand Free Fluid Index Determination,” by Miller et al., SPE 20561, 65thAnnual Technical Conference of the SPE, New Orleans, La., Sep. 23-26,1990; “Improved Log Quality With a Dual-Frequency Pulsed NMR Tool,” byChandler et al., SPE 28365, 69th Annual Technical Conference of the SPE,New Orleans, La., Sep. 25-28, 1994). Details of the structure, theoperation and the use of the MR tool are also discussed in U.S. Pat.Nos. 4,717,876; 4,717,877; 4,717,878; 5,212,447; 5,280,243; 5,309,098;5,412,320; 5,517,115; 5,557,200 and 5,696,448 all of which are commonlyowned by the assignee of the present invention. The Schlumberger CMRtool is described, for example, in U.S. Pat. Nos. 5,055,787 and5,055,788 to Kleinberg et al. and further in “Novel NMR Apparatus forInvestigating an External Sample,” by Kleinberg, Sezginer and Griffin,J. Magn. Reson. 97, 466-485, 1992. The interested reader is directed tothe disclosure of these patents for relevant background information.Accordingly, the patents listed above are hereby incorporated byreference for all purposes.

Referring to FIG. 1, it illustrates an improved NMR MWD tool 40 used andoperated in accordance with a preferred embodiment of the presentinvention. NMR tool 40 comprises a non-magnetic metal drill collar 10that encloses a tubular, permanent magnet 12, which surrounds a mud tube34 through which drilling mud may be pumped during the drilling of aborehole 36 into the earth formation 138. The tool 40 further comprisesa stabilizer section 66, antenna 14, magnet section 12, capacitorcompartment 32 with tuning capacitors 42, accelerometers 24, antennadriver 28, signal processor 30 and data transmitter 150. Some of thesecomponents are known in the art and have been described generally, forexample, in the prior art publications discussed in the precedingsection, which are incorporated by reference. In the sequel, variousimprovements to these components of the tool and their functions aredescribed in more detail in accordance with the preferred embodiments ofthe present invention.

(1) The Magnet

In accordance with the present invention, the magnet 12 of NMR tool 40is polarized such that a static magnetic field B₀ illustrated in furtherdetail in FIG. 6B, is produced in the earth formation 138 that issubstantially transversally oriented with respect to the longitudinalaxis 5 of the tool. As shown in FIG. 6B, the magnetic field of the toolin accordance with the present invention can be modeled as that of alinear dipole. The magnet 12 is preferably fabricated from highly stablerare-earth material combinations, such as samarium-cobalt orneodymium-iron-boron. Custom-shaped blocks can be obtained, for example,from Magnetfabrik Schramberg, Schramberg, Germany.

In a preferred embodiment, the length of the magnet 12 is about fourfeet (1.2 meters), of which in a preferred embodiment only the centertwo feet (0.6 meters) are used for the NMR measurement. In accordancewith the present invention, the additional one-foot sections at each endprovide a way to pre-polarize the hydrogen nuclei in the formation andto establish nuclear polarization equilibrium before a portion of theformation enters the measurement volume. Due to the symmetric nature ofmagnet 12 and the measurement volume for the tool built in accordancewith the present invention, the NMR response is independent of thedirection (i.e., upward or downward) in which the tool moves.

As best seen in FIGS. 4 and 19, the magnet 12 in a preferred embodimentcomprises a plurality of magnet segments 58. Axially, the magnetsegments 58 are separated by magnet spacers 64, preferably made ofsilicone foam, which are installed with a compressive preload.Circumferentially, the magnet segments 58 are bonded to an inner magnetsleeve 60 and an outer magnet sleeve 62. In a specific embodiment, eachaxial section of magnet segments is held in proper circumferentialalignment by means of a key 54 pressed into a longitudinal slot formedin the mud tube 34 and inner magnet sleeve 60. Thus, in a preferredembodiment the magnet 12 is segmented both axially andcircumferentially. The axial segmentation provides improved static anddynamic loadhandling capability. The magnet 12 is installed over the mudtube 34, and a plurality of O-rings 56 are installed in the small gapbetween the mud tube 34 and inner magnet sleeve 60. The O-rings 56provide a means for handling radial expansion, isolate the magnet fromdistortions of mud tube 34, and also help to hold the magnet 12 in placeaxially.

In the preferred embodiment, the magnet segments are made fromsamarium-cobalt, although for temperatures below about 160° C.alternative embodiments, such as neodymium-iron-boron segments wouldalso be a suitable and less expensive choice. In accordance with thepresent invention, the shaping of the magnetic field outside the tool isaccomplished inpart by grinding the individual samarium-cobalt segmentsinto individual shapes and volumes. In particular, ferrite materiallayers are 18 mounted in a preferred embodiment over the north and southsides of the magnet and have the effect of partially short-circuitingthe static magnetic flux. In order to achieve a magnetic field ofuniform and sufficient magnitude within the sensitive volume, in apreferred embodiment it is necessary to: (a) slightly increase thevolume of all magnetic segments, and (b) to selectively increase thesegment volume on the north and south sides at the expense of east andwest sides, because it is the former sides that are mostly affected bythe presence of ferrite layers 18.

A modification to the cross section 4—4 in FIG. 1, as shown in aspecific embodiment in FIG. 4 is illustrated in FIG. 4C. The arrangementillustrated in FIG. 4C is particularly advantageous because it achievesthe mechanical strength of the embodiment shown in FIG. 4, combined witha simplified geometry for the magnet 12. In the embodiment illustratedin the figure, the magnet 12 is comprised of magnetized segments 58 madefrom samarium-cobalt. These magnetized segments are bonded to the flatsides of a magnet segment carrier 50, which is made from magneticallypermeable steel (type 4130). In accordance with the present invention,the magnet segment carrier 50 is hollow to accommodate the mud tube 34,which encloses the flow channel. The orientation of magnetization isgenerally along the N-S direction, as indicated in FIG. 4C. The magnetsegment carrier 50 generally becomes magnetized in the same N-Sdirection. The aforementioned pre-compensation of the magnetic field iseasily accomplished, for example, by shortening or elongating the magnetcarrier 50 along the N-S and/or along the E-W direction, respectively.

(2) Ferrite Material Layers

In order to improve the performance of the antenna 14 of the tool in thereceive mode as well as the transmit mode, in accordance with apreferred embodiment of the present invention an axially elongated layerof ferrite material 18 is installed between the shield 16 and theantenna 14, as shown in FIGS. 4, 4A, 4B, 4C and 4D. Generally, theferrite material layer 18 shapes the radio frequency (RF) field, byoffsetting the reduction in the antenna aperture due to shield 16.Without the ferrite layer 18, large eddy currents would be induced inshield 16, which would tend to oppose the antenna currents and wouldresult in significantly reduced sensitivity in receive mode and in muchlarger current and power requirements in transmit mode.

Further, it is important to note that the field generated by the magnet12 is not rotationally symmetric in its amplitude. Rather, in apreferred embodiment, the design and construction of the magnet takesinto account the effect of the soft-magnetic ferrite material layer 18,which is mounted over both the north and the south poles of magnet 12.Ferrite material must be used that does not magnetically saturate in thepresence of the strong static field surrounding the magnet. Basematerials for ferrites 18 that can be used in a preferred embodiment are3F3 and/or 3F4. Although the ferrites 18 are employed in accordance withthe present invention to shape the radio frequency field, it will beappreciated that they also distort the static magnetic field.Accordingly, in the present invention the main magnetic field can bepre-compensated such that the combined field from the magnet and theferrites is essentially rotationally symmetric in its magnitude withinthe measurement volume. In a preferred embodiment this can beaccomplished by selectively altering the volume of magnetic material onthe north and south sides vs. the amount on the east and west sides ofthe magnet. Alternatively, or in combination with the above approach,the size of the ferrites can also be optimized for length and width toincrease absorption and/or destructive reflections. In a specificembodiment the ferrites 18 can also be staggered like a brick wall forthe same purpose. The required modifications of the magnet shapes arepreferably determined by numerical modeling, such as provided bycommercial packages, of which ANSYS is an example.

As shown in the detail FIG. 4D, in a preferred embodiment of the presentinvention the ferrites 28 are actually comprised of layers of ferritematerial, bonded by lead impregnated epoxy resin. In a preferredembodiment, the ferrite material layers are about 0.050″ thick, whilethe epoxy resin is about 0.015″ thick. The layers are oriented such thata more or less continuous path exists for the magnetic component of theRF field. In such an arrangement, the RF magnetic field experienceslittle attenuation as it passes through the ferrite. On the other hand,magneto-acoustic vibrations, which are introduced by the RF pulse, areoptimally dampened because the acoustic wave repeatedly experiencesabsorption and/or destructive reflection on the interfaces between theferrite material and the bonding layers.

As known in the art, commercially available ferrite is physically porousas well as magnetically permeable and is therefore prone to destructionunder the typically high borehole pressures. Additionally, nonisostaticpressure changes the permeability of ferrite, which in turn changes theresonant frequency of the antenna 14. Therefore, the ferrite 18 ispreferably impregnated with epoxy resin under pressure and temperatureas described, for example, in U.S. Pat. No. 5,644,231 to Wignall. In thealternative, the ferrite layer 18 can be prepared by hot isostaticpressure methods to press out all the voids in the ferrite. As with theinterface between the drill collar 10 and the shield 16, the preferredmethod of mounting the ferrite layer 18 to the shield 16 in accordancewith the present invention is by bonding a thin layer 112 (FIG. 4A) ofrubber or lead-filled epoxy between the shield 16 and ferrite layer 18.This thin layer 112 of rubber or lead-filled epoxy serves toacoustically decouple the shield 16 from the ferrite layer 18.

(3) The Antenna

In accordance with the present invention, the length of the measurementvolume is defined by the aperture of the antenna 14 and in a preferredembodiment is about two feet (0.6 m) long. In the preferred embodimentof the tool illustrated in FIG. 1 the antenna is used both for thetransmission of radio frequency (RF) pulses into the surroundingformation 138 and for receiving NMR signals from the formation. Inaccordance with the present invention antenna 14, also illustrated in aschematic view in FIG. 5, is preferably made of flat, elongated copperstrips 14A interconnected by peripherally extending copper strips 14Babout 1 inch wide and about 0.030 inch thick, and is mounted on theexternal surface of the drill collar 10 along the same axial portion ofthe tool as the magnet 12. The antenna is fed by a transmission line 14Cmade from two copper strips separated by a thin layer of suitabledielectric material. In a preferred embodiment, antenna 14 is completelyencapsulated with a protective coating, which is preferably made ofvulcanized viton rubber or a thermoplastic composite material.

Antenna-Magnet Coupling

In accordance with the present invention it is very important to providesemi-rigid coupling between antenna 14 and permanent magnet 12. If thisis not the case, relative movement of the antenna against the staticmagnetic field will induce an electric voltage in the antenna windings.Although such a voltage would be small, it could still be large comparedwith the voltages induced by NMR signals, and would have a deleteriouseffect on the accurate measurement of the latter. As can be seen in FIG.4, the magnet 12 is rigidly coupled to the drill collar 10 usinglocking.

Despite the potential deleterious effect on the NMR signal, it has beendemonstrated by experimentation that it is still advantageous topartially de-couple the antenna from the drill collar 10 through shield16 and antenna mounting 20. Generally, this de-coupling is believed toprovide a measure of isolation from the effects of dynamic geometrychanges, such as cyclical bending and torsion variations in collar 10,and high-frequency vibrations created by the drilling process andtransmitted through the collar.

Mounting of the Antenna

The best known method for mounting the antenna 14 onto the drill collar10 is by first installing an antenna mounting 20 made of an electricaland structural insulator material, such as a fiber-reinforced epoxy orthermoplastic composite, onto the ferrite 18 using another thin bondinglayer 112 (See FIG. 4A). A recess 20A is formed in the outer surface ofthe antenna mounting 20 for accepting the antenna 14, which is bonded tothe antenna mounting 20 with yet another thin bonding layer 112.Alternatively, as shown in FIG. 4B, the antenna 14 may be installeddirectly to the ferrite layer 18, without an intervening antennamounting 20, using a thin bonding layer 112. In such case, a recess 18Ais formed in the ferrite layer 18 for accepting the antenna 14. Ineither case, the antenna 14 is preferably covered with an antenna cover22 in the form of a replaceable sleeve to protect the antenna 14 fromthe borehole environment. The preferred material for the antenna cover22 is also a fiber-reinforced epoxy or thermoplastic composite. Withfurther reference to FIG. 1, in a preferred embodiment the antenna cover22 is held in place by a stabilizer 66 installed at each end of theantenna cover 22. These stabilizers 66 are preferably made of shrink-onmaterial that allows installation by heating the stabilizers, slidingthe stabilizers onto the drill collar 10, and allowing the stabilizers66 to contract, or shrink, upon cooling. In addition to holding theantenna cover 22 in place, the stabilizers, which have an outer diameterlarger than that of the antenna cover 22, serve to minimize damage tothe antenna cover 22 due to rubbing and bumping against the boreholewall 38.

When installed in this manner, the antenna 14, shield 16, ferrite 18,and magnet 12 cooperate electromagnetically to produce an annular NMRsensitive volume 36, as shown in FIGS. 6A and 6B. In a preferredembodiment, the sensitive volume 36 has a nominal diameter D_(sv) ofabout 13.5 inches (0.34 m), a radial thickness t_(sv) of about 1.5 mm,and an axial length of about 2.0 feet (0.6 m). With reference to FIG. 1,this relatively large nominal diameter allows the sensitive volume to beentirely within the formation 138, yet the antenna is efficient enoughto accurately detect the NMR signals at this distance. Additionally, thestatic magnetic field of this invention as a function of radial distancefrom the tool is such that the location of the larmor frequency for thesodium quadruple moment lies inside the tool volume instead of in theborehole 36 or in its wall 38. Therefore, in an important aspect of thisinvention, sodium signal is prevented from developing in the boreholefluid.

As illustrated in a specific embodiment in FIG. 4, the diameter ofantenna cover 22 is slightly less than the largest tool diameter. Thelargest tool diameter is defined by metallic stabilizers (66) that aremounted above and below the sensor section of the tool. If nostabilizing action is desired, hardened wear bands may be substitutedfor the stabilizers. The beneficial effect of stabilizers and/or wearbands used in accordance with the preferred embodiment is that theantenna cover 22 typically does not come into contact with the boreholewall 38. Wear bands are replaceable in the field and can be renewedbetween runs into the hole. It is also envisioned that metallic rings,such as stabilizers and/or wear bands, are placed immediately adjacentto the main antenna, such that in operation only the center two feet ofantenna cover 22 remain exposed.

(4) The Drill Collar

FIGS. 4 and 4C, which show cross-sections along line 4—4 in FIG. 1, showtwo different embodiments of the drill collar 10 used in accordance withthe present invention. Specifically, in the embodiment shown in FIG. 4,the collar 10 has a “wing-shape” cross section. FIG. 4C, on the otherhand, illustrates an embodiment with an octagonal cross-section design.Generally, it is important to note that the ferrites 28 are shaped toconform to the outer surface of the drill collar. In the embodimentillustrated in FIG. 4C an electromagnetic shield 16 is realized byplating the drill collar, although an arrangement as shown in FIG. 4A isalso possible in an alternative embodiment. Numerous different drillcollar cross-section designs are possible in accordance with the presentinvention, where each variation embodies a particular compromise betweenthe strength of the steel collar 10 and the efficiency of the antenna14.

(5) Conductive Shielding

Referring next to FIGS. 4, 4A-D, in accordance with a preferredembodiment of the present invention a highly conductive shield 16,preferably made of copper, is used between the antenna 14 and theexterior of the drill collar 10. The purpose of this shield is toprevent the RF field from entering the conductive steel collar 10.Otherwise, it will be appreciated that the steel collar would reduce theQ factor of the antenna 14, resulting in diminished signal amplitude inreceive mode, and increased power dissipation in transmit mode. Inaccordance with the present invention the shield 16 is preferably a“floating” equal-potential surface with no direct connection to thecollar.

The conductive shield 16, which completely surrounds the drill collar 10along the axial portion containing the antenna 14, also helps to reducethe ringing of the drill collar 10 due to the strong RF pulses used inNMR measurements. The preferred method for installing the shield 16 ontothe drill collar 10 in accordance with the present invention is bybonding a thin layer 112 (FIG. 4A) of material having suitable acousticproperties, such as rubber or lead-filled epoxy, between the collar 10and the shield 16. In alternative embodiments the shield can beinstalled using different methods, such as flame spraying,electroplating, by means of mechanical fasteners, or otherwise. Inaccordance with the present invention the acoustic decoupling materialcomprising the bonding layer 112, such as rubber or lead-filled epoxy,is desirable as a dampening material for two reasons: (1) it is softenough to acoustically decouple the shield 16 from the drill collar 10,and (2) it is rigid enough to prevent the shield 16 from vibrating dueto the interaction between the current in the antenna 14 and the staticmagnetic field.

(6) The Tuning Capacitor Compartment

Referring back to FIG. 1, this preferred embodiment also incorporatestuning capacitors 42 housed in a tuning capacitor compartment 32 of thetool 40. The tuning capacitors 42 are used to match the impedance of theantenna 14 so that it will resonate at the desired natural frequency. Asdescribed, for example, in U.S. Pat. No. 5,557,201, the tuning capacitorcompartment 32 is sealed off from the borehole environment so that thecapacitors remain at atmospheric pressure instead of being exposed tothe high borehole pressures. This pressure-sealed design eliminates theneed for filling the capacitor compartment 32 with oil, as in prior art,to prevent the capacitors from contacting borehole fluids. Additionally,this invention comprises a high-pressure antenna feed-through connector52, as shown in illustrative embodiments in FIGS. 18 (corrugatedcross-section), 18A (multi-finned cross-section) and 19, to provide aconductive path for the electrical current from the antenna 14 to thetuning capacitors 42. The feed-through connector aspect of thisinvention is discussed in a preferred embodiment in an application filedconcurrently herewith.

In accordance with the present invention, by maintaining the capacitorcompartment at atmospheric pressure, more pressure-sensitive electronicsmay be mounted inside the tuning capacitor compartment. This includes,but is not limited to, electromechanical relays and associated driverelectronics. Under control of the driver electronics, such relays can beused to add more tuning capacitors to the resonant circuit formed by thefixed capacitors and the antenna. Thereby, the resonant frequency of theresonant circuit can be changed and the system can be made to operate atdifferent frequencies one at a time. Such an arrangement is advantageousbecause by changing the operating frequency, a different sensitivevolume is selected. By using multiple volumes one at a time, more signalcan be accumulated in less time and/or different NMR measurement can beperformed in a quasi-simultaneous fashion. Reference is made here to thepaper “Lithology-Independent Gas Detection by Gradient NMR Logging,” byPrammer, Mardon, Coates and Miller, Society of Petroleum Engineers,paper SPE-30562, published in the transactions to the 1995 SPE AnnualTechnical Conference & Exhibition, pp. 325-336, which is herebyincorporated by reference. In FIG. 6 of this paper, a pulse sequence foran NMR wireline tool is shown that utilizes two measurement volumes atonce to affect oil and gas detection.

(7) Power Generation

FIG. 24 illustrates an electric generator 80 in accordance with thepresent invention for supplying electrical energy to a downhole system140. Electric generator 80 is driven by a drive shaft 86 that ispreferably connected to a conventional mud-powered turbine (not shown)and supported by bearings (not shown). Electric generator 80 comprisespermanent magnets 82 and 84, which are preferably of equal length andmagnetic strength and which rotate inside a fixed main armature 88 togenerate downhole electric energy. Because such electric energy isneeded over a widerange of rotation speeds of drive shaft 86 (i.e., themud-powered turbine) and electrical demands of system 140, theelectrical output must be controlled. The present invention controls theelectrical output by providing a regulator for varying the relativerotational position of movable magnet 84 with respect to fixed magnet82. Specifically, fixed magnet 82 is fixedly attached to drive shaft 86,but movable magnet 84 is mounted to a carriage 114 that is mounted todrive shaft 86 with a bearing 120 such that carriage 114 may rotate withrespect to drive shaft 86. The degree of relative rotation betweencarriage 114 and drive shaft 86 is preferably limited by a stop pin 104as discussed below.

As will be readily apparent to persons skilled in the art, the presentinvention may be used to generate AC or DC electrical energy. If thisinvention is used to generate DC electrical energy, a rectifier 142 isprovided as shown in FIG. 24 to rectify the output from main armature 88before it is fed into controller 106 and on to system 140.

As shown in FIGS. 25, 26, 27, 28A and 28B (in which main armature 88,carriage 114, bearing 120, and drive shaft 86 are not shown forclarity), magnets 82 and 84 comprise a plurality of longitudinalpermanent magnet segments, which are preferably bonded to carriage 114.The magnetization of the magnet segments alternates circumferentiallyfrom north pointing radially outward to north pointing radially inward.When magnets 82 and 84 are completely aligned as shown in FIG. 28A, themaximum electrical output will be generated. Conversely, when magnets 82and 84 are completely misaligned as shown in FIG. 28B, zero electricaloutput will be generated. For the preferred embodiment shown, this rangeof movement is 45° (angle θ in FIG. 26). Thus, the requisite amount ofelectrical output is achieved by positioning magnets 82 and 84 betweenthese two extremes. As shown in FIGS. 24, 26 and 27, a preferredembodiment limits this range of motion to the appropriate degree bymeans of a stop pin 104 that rotates within a transverse cavity in theform of a pair of symmetric sectors 86B within an enlarged portion 86Aof drive shaft 86. A biasing element 100, preferably of hexagonalcross-section, is installed through an axial cavity 86C in one end ofdrive shaft 86 and into a matching, preferably hexagonal, shaped hole instop pin 104. Biasing element 100 serves to bias carriage 114 andmovable magnet 84 in the maximum-output position with stop pin 104against one extreme of sectors 86B. This biasing effect is accomplishedby applying a torsional preload on biasing element 100 in the directionof the rotation of drive shaft 86 and securing biasing element 100 inthe preloaded position with a set screw 116 contained in an end fitting118. Stop pin 104 protrudes through a hole in carriage 114 and therebyrotates with carriage 114 when carriage 114 is rotated by a drag torque,as discussed below, After carriage 114 has been rotated from its initialposition with respect to drive shaft 86 by means of a drag torque asdiscussed below, stop pin 104 serves to return carriage 114 to itsinitial position by means of biasing element 100.

Persons skilled in the art will recognize that the hexagonal shape ofbiasing element 100 and the corresponding hole in stop pin 104 aresimply a convenient means of fastening biasing element 100 to stop pin104 using a segment of a conventional hex key (Allen wrench). Ingeneral, the shape need not be hexagonal so long as another means offastening biasing element 100 to stop pin 104 is provided. Furthermore,the means of biasing carriage 114 and movable magnet 84 toward a certainposition could take a variety of other forms, such as a coil spring.Moreover, the biasing mechanism could be located outside rather thaninside drive shaft 86 if, for instance, electrical wires need to berouted through the inside of drive shaft 86. Also, although a preferredembodiment comprises a biasing mechanism, a biasing mechanism is notabsolutely necessary for all applications and could be eliminated, ifdesired.

Persons skilled in the art will also recognize that the configuration ofmagnets 82 and 84 and the relative rotation limiting device for varyingthe amount of electrical output generated by an embodiment of thisinvention may take a variety of other forms. For example, the relativerotation may be limited to less than that which would be required toachieve complete misalignment of magnets 82 and 84 such that the maximumallowable rotation produces a certain fraction of maximum output insteadof zero output. Alternatively, magnets 82 and 84 may be made of unequalaxial length such that rotation into the completely misaligned positionproduces a certain fraction of maximum output instead of zero output.Additionally, the number of magnet segments comprising magnets 82 and 84may be varied such that a rotation angle other than 45° is required toachieve complete misalignment. As shown in FIG. 25, the cross-sectionsof magnets 82 and 84 preferably have a circular outer shape and apolygonal inner shape. A circular outer shape is preferable forproviding an optimal magnetic field to cooperate with main armature 88,and a polygonal inner shape is preferable for ease of manufacture and tohelp prevent the magnet segments from de-bonding from carriage 114 dueto torsional loads. However, the outer and inner shapes of magnets 82and 84 may comprise other suitable shapes, as will be readily recognizedby persons skilled in the art. Because magnets 82 and 84 may be ofpolygonal cross-section or circular cross-section, the term“circumferential” as used herein to describe magnets 82 and 84 should beunderstood to mean the peripheral dimension of those elements, whetherflat or curved. Also, although the magnet segments comprising magnets 82and 84 are preferably of equal circumferential dimensions, they may beof unequal circumferential dimensions, if desired.

Further, the regulator for varying the position of movable magnet 84with respect to fixed magnet 82 may take a variety of forms. Referringto FIG. 24, a preferred regulator comprises a drag element 98 mounted toa bearing 102 on drive shaft 86. Drag element 98, which rotates inside afixed drag armature 90, is preferably made of copper and serves as apath for developing an eddy current. It should be understood that copperis referred to as a preferred material for certain elements of thisinvention, but any suitable conductive material could be used in placeof copper for such elements. In typical downhole operations,fluctuations in parameters such as input RPM, electrical demands ofsystem 140, and ambient temperature tend to cause fluctuations in theelectrical output from main armature 88. Therefore, a preferredregulator includes a controller 106 which contains suitable electronicsfor monitoring the electrical output from main armature 88 and makingappropriate adjustments to the input to drag armature 90, as discussedbelow, to modify the electrical output from main armature 88 and therebymeet the electrical requirements of system 140. Specifically, controller106 generates an appropriate electrical control current in the windingsof drag armature 90, which sets up a first magnetic field. The rotationof drag element 98 within the first magnetic field creates an eddycurrent in drag element 98, which is a function of (1) the magneticfield created by drag armature 90, (2) the speed of rotation of dragelement 98, (3) the conductivity of drag element 98, and (4) the axiallength of drag element 98. In turn, the eddy current in drag element 98produces a second magnetic field that opposes the first magnetic field,which creates a drag torque on drag element 98. Thus, drag element 98(rotor) and drag armature 90 (stator) function as a drag torquegenerator. The drag torque causes drag element 98 to rotate relative todrive shaft 86 in the direction opposite that of the drive shaftrotation. Because drag element 98 is connected to movable magnet 84through a torque converter as discussed below, the drag torque rotatesmovable magnet 84 relative to fixed magnet 82 by an appropriate amountaccording to the applied electrical control current. The relativemovement of movable magnet 84 with respect to fixed magnet 82 modifiesthe electrical output from main armature 88. Thus, as controller 106senses deviations in the output from main armature 88, controller 106makes appropriate modifications to the electrical control current indrag armature 90 to cause appropriate modifications to the output frommain armature 88 and thereby meet the electrical requirements of system140.

Persons reasonably skilled in the art will recognize that the requireddrag torque may be generated by a variety of other rotor/statorconfigurations, such as: (1) a copper drag element rotating insidepermanent magnets housed in a fixed armature; (2) permanent magnetsrotating inside a fixed copper cylinder; (3) a copper drag elementrotating inside a motor-driven, rotatable drag armature comprising aseries of alternately magnetized permanent magnet segments, similar tomagnets 82 and 84 as shown in FIG. 25, which can be rotated in eitherdirection to advance or retard the drag element, as appropriate; or (4)a drag element, comprising a series of alternately magnetized permanentmagnet segments similar to magnets 82 and 84 as shown in FIG. 25,rotating within a stator comprising windings which can be energized tocontrol the speed and direction of a rotating magnetic field and thusadvance or retard the drag element, as appropriate. The foregoingoptions (1) and (2) would not include a controller 106 and thereforewould not be responsive to the output from main armature 88; rather,those two open-loop options would be responsive only to changes in driveshaft speed and would simply limit the output from main armature 88. Bycontrast, the latter two options (3) and (4) would provide an additionaladvantage of helping to reduce the time that the apparatus takes toreturn to the initial, maximum-output position by enabling theapplication of a “reverse” drag torque (i.e., a torque in the samedirection as the rotation of drive shaft 86) to drag element 98, therebyassisting biasing element 100 in moving carriage 114 and movable magnet84 back to their initial position. If desired, option (1) or (2) couldbe used in conjunction with the other drag torque generatorconfigurations described herein to provide both a rudimentary limit tothe output and a more sophisticated output control mechanism. Therudimentary limit provided by option (1) or (2) in such a hybridconfiguration may be desirable, for example, to prevent an electricaloverload in the event of failure of the electronics in controller 106.Of course, the drag torque could also be supplied by a mechanical brake.

To achieve the desired movement of carriage 114 with as small a dragtorque as possible, the drag torque generated on drag element 98 ispreferably multiplied using a torque converter as it is transmitted tocarriage 114. In a preferred embodiment, the torque converter comprisesa harmonic drive mechanism such as those sold by Harmonic DriveTechnologies, Inc. and HD Systems, Inc. Alternatively, the torqueconverter could comprise other known gear mechanisms, such as aplanetary gear mechanism. Although it may be possible to eliminate thetorque converter in certain embodiments of this invention, the absenceof a torque converter would increase the input torque requirements tounacceptable levels in most instances.

As shown in FIG. 24, a preferred harmonic drive mechanism comprises awave generator 92, a flexspline 94, and a circular spline 96. Dragelement 98 is fixedly connected to wave generator 92, and circularspline 96 is fixedly connected to carriage 114 which comprises movablemagnet 84. Circular spline 96 is relatively stiff and has internal teethto engage flexspline 94. Flexspline 94, which is of slightly smallerdiameter than circular spline 96 and has fewer teeth (usually two fewer)than circular spline 96, is relatively flexible and has external teethto engage circular spline 96. Wave generator 92 comprises an elliptical,thin raced ball bearing that fits inside flexspline 94 and causesflexspline 94 to engage circular spline 96 at each end of the major axisof the ellipse. Wave generator 92, flexspline 94, and circular spline 96cooperate such that each revolution of wave generator 92 causes circularspline 96 to rotate by only two teeth, for example. Thus, the dragtorque on drag element 98 is multiplied as transmitted to carriage 114and movable magnet 84 as a control torque. A tradeoff for achieving thistorque multiplication is that the harmonic drive mechanism increases theresponse time of the apparatus. However, if desired, the use of amotor-driven, rotatable drag armature as mentioned above would help todecrease the response time.

Because power generator 80 comprises a brushless, noncontact apparatus,it has an additional advantage of being capable of operating whileimmersed in oil. Thus, if oil is needed for pressure balancing due tohigh downhole pressures, this generator can safely operate in anoil-filled compartment.

Persons skilled in the art will recognize that other advantageousconfigurations are possible to vary the amount of electrical outputgenerated by an embodiment of this invention. For example, anadvantageous configuration may be to fix the initial relationship ofmagnets 82 and 84 in a certain degree of misalignment such that thedefault electrical output is somewhat less than the maximum possibleoutput. Indeed, it may be beneficial in certain applications to have aninitial relationship of complete misalignment of magnets 82 and 84 suchthat the initial electrical output is zero. By selecting the properarrangement of the harmonic drive output direction and the direction ofthe biasing torque, the drag torque could be made to increase ordecrease the electrical output, as desired. However, in a preferredembodiment of this invention, controller 106 is powered by a portion ofthe output from main armature 88. Therefore, an initial relationship ofcomplete misalignment of magnets 82 and 84 which produces zero initialoutput generally would not be desirable unless an alternate power sourceis provided for controller 106.

Still another advantageous configuration may be to have a threadedcooperation of movable magnet 84 on drive shaft 86 such that the dragtorque created on drag element 98 translates movable magnet 84 axiallyand thereby changes the electrical output by changing the percentage ofmovable magnet 84 that is encompassed by main armature 88. Such athreaded configuration would also vary the electrical output by changingthe separation distance between fixed magnet 82 and movable magnet 84.

Because the present invention is intended to be able to operate atelevated downhole temperatures, the various magnets referred to hereinpreferably comprise samarium-cobalt (Sm—Co) magnets. Although some othertypes of magnets, such as neodymium-iron-boron (Nd—Fe—B) magnets,generally provide better magnetic flux at lower temperatures, Sm—Comagnets maintain better energy density at temperatures above about 150°C. However, any suitable type of magnets may be used, if desired.

(8) Electronics

Referring next to FIG. 33, it illustrates in a block-diagram form thepreferred electronics arrangement for the tool of the present invention.In particular, the resonant circuit comprised of antenna 14 and thetuning capacitors 32 is interfaced to a transmit/receive switch 330.This T/R switch receives pulsed RF power from the transmitter 355, whichis gated by the pulse generator 350. The pulse generator is undercontrol of the computer/signal processor 30. The T/R switch 330 routesthe received signal to a preamplifier 335, which in turn drives thereceiver section 340. The received, amplified signal is digitized indigitizer 345 and fed into the processor 30.

In a preferred embodiment, processor 30 receives real-time motion datafrom the motion sensor interface 310, which conditions the electricalsignal from motion sensors 24. The operation of the motion sensors isdescribed in frther detail below. Additionally, the processor reads fromand writes to a non-volatile data and program memory 315. In a preferredembodiment, this memory retains data even when the electronics is notsupplied with electrical power. In a preferred implementation, thenon-volatile memory uses “Flash” EEPROM integrated circuits. Anothersuitable option is a battery-powered low-power CMOS static RAM. Thememory 315 holds all data acquired during a run. Processor 30 performsreal-time processing on the data to extract an indication of formationporosity and of log quality. In a preferred embodiment, this data isconverted into a data stream of preferably very low bit rate and are fedinto a mud-pulse system 320 that broadcasts the data stream to thesurface by means of pressure pulses within the fluid column within thedrill collar. Above-surface processing equipment (not shown) can be usedto display the results to an operator. It will be appreciated thatdifferent tool-to-surface communication approaches are possible inalternative embodiments. Further, those skilled in the art willappreciate that downhole processor 30 may be implemented using two ormore dedicated signal processors communicating with each other. In thisembodiment, each processor can be performing a different task. Forexample, with reference to the following section on motion detection, adedicated processor can be used to monitor signals related to the motionof the tool in the borehole and to provide signals for definingappropriate time windows when NMR signals from the formation are to beprocessed by a separate signal processor. It is applicants' intentionthat any suitable processor configuration can be used in accordance withthe principles of the present invention. Further, it should be apparentthat various options that exist for storage and communication of theacquired information to the user can be used without departing from thespirit of this invention.

The electrical power required to operate the tool was described in somedetail in the preceding section. FIG. 33 illustrates the powergeneration in a block diagram form. In particular, as shown this poweris derived in accordance with the present invention from one or more ofthe following sources: (a) an (optional) bank of primary battery cells89, typically of the lithium type, or (b) from a turbine/generatorcombination 80-87 that converts a portion of the mechanical energydelivered by the flowing mud column into electrical energy. Thegenerator can be used to directly drive the power conversion unit 360. Apotential disadvantage of this arrangement is that the tool cannotoperate without mud being continuously pumped from the surface throughthe drill collar through the NMR tool to the drill bit. This requirementcould potentially interfere with the requirements of the drillingoperation. Therefore, in a preferred embodiment, the turbine/generatorcombination is used to charge a bank of rechargeable secondary batterycells 81, for example of the nickel-cadmium or silver-oxygen type. In apreferred embodiment, the generator 80 is sufficiently powerful torecharge the secondary elements in a short amount of time, while thesesecondary cells supply electric power to the tool during the time whenno or very slow mud flow exists.

The power conversion unit 360 converts the energy from primary orsecondary cells into a form suitable for short-term storage inhigh-voltage capacitors 365. These capacitors feed the transmitter powersupply 370 and are capable of discharging within fractions ofmilliseconds as required to generate high-power RF pulses of shortdurations.

(9) Auxiliary Antennas and Their Functions

Another important aspect of the present invention is the use of one ormore auxiliary coils 144 as shown in FIG. 33 and FIG. 5. These coils aremounted in one or more recesses within the aperture of the main antenna14, where a small amount of the RF flux from the main antenna is sampledby the auxiliary coil(s) 144. Coil(s) 144 act as small antennas and areinterfaced to a calibration circuit 325 (FIG. 32), which is undercontrol of processor 30. The purpose of coil(s) 144 is three-fold:Firstly, during transmission of RF pulses, coil 144 picks up a fractionof the RF magnetic flux and generates a proportional voltage. Thisvoltage is amplified by the electronics and generates an indicatorsignal for the strength of the outgoing RF pulse. This information isfed back into the driver electronics for the RF pulse generation toincrease or to decrease the delivered RF pulse power. Thus, inaccordance with the present invention a constant RF pulse amplitude isachieved that is independent of variable load conditions due to changesin borehole size, changes in the conductivity of borehole fluids and/orchanges in conductivity of the formation.

In accordance with the present invention the auxiliary antennas 144 haveanother purpose: Before an NMR pulse sequence is started, a referencesignal of known amplitude is injected into the system by means ofcoil(s) 144. The resultant magnetic flux is picked up by the antenna 14,amplified and processed by the signal processor. This reference signalacts in a preferred embodiment as built-in secondary calibration. Inparticular, by comparing the signal amplitudes received during the CPMGsequence to the apparent signal strength of the reference signal, it ispossible to derive signal amplitudes which are independent of the systemgain factor and of the Q factor of the resonant circuit formed by theantenna and the resonant capacitor(s). The reference signal is typicallytransmitted once per second through coil(s) 144.

Furthermore, a potentially dangerous condition can be detected bymonitoring the apparent strength of the reference signal generated bythe auxiliary antennas. Specifically, a low reference signal istypically caused by excessive loading of the antenna 14, for instance bythe presence of metallic pipe lining the borehole walls and surroundingthe NMR tool. Transmitting an RF pulse in such a situation couldpotentially damage the RF transmitter by reflecting the outgoing energyback into the tool. Therefore, in accordance with a preferredembodiment, a timing circuit is triggered when an excessive loadcondition exists and disables the transmitter for about 15 more minutesafter the load condition is removed. This is a fail-safe mechanism usedin a preferred embodiment to make sure that the tool is never in thevicinity of casing when the transmitter is turned on.

The fail-safe mechanism described above is also used in accordance withthe present invention to make sure that the tool never transmits RFenergy while in free air on the surface. Such a transmission may bedisruptive and potentially hazardous for nearby electronic circuits.Accordingly, a protective, conductive blanket is provided, that theoperator wraps around the tool whenever the tool is on the surface. Thetool detects this condition once every second and goes into fail-safemode with the transmitter turned off. After the conductive blanket isremoved, in a specific embodiment the operator has about 15 minutes todeploy the tool into the borehole. Typically, the borehole near thesurface is cased, which keeps the RF transmitter turned off as long asthe tool resides within the cased portion of the borehole.

Finally, the periodic signal radiated by auxiliary antenna(s) 144provides in accordance with the present invention a convenient means forverifying the operation of processor 30 without having to makeelectrical contact with the tool.

(B) NMR Measurement Methods Using Motion Management

NMR measurement methods are generally known and are described in avariety of prior art references including, for example, U.S. Pat. Nos.4,710,713; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115,5,557,200 and 5,696,448 to the assignee of the present invention. Thesepatents are incorporated herein by reference for all purposes. Thefollowing discussion therefore focuses on aspects of the NMR measurementmethods relevant in the context of the present invention.

The preferred method for the NMR measurement in accordance with thepresent invention is to use the Carr-Purcell-Meiboom-Gill pulse sequence(CPMG) as shown in FIGS. 33(a) and 33(b). The first pulse in thesequence (90°-pulse, with 90° phase) is typically 50 μsec long, allsubsequent pulses (180°-pulses, with 0° phase) are typically 100 μseclong. The timing is such that the center-to-center delay between thefirst and the second pulse (τ) is 250 μsec and all other pulses are 500μsec center-to-center apart. With such a pulse timing, NMR echoes can bedetected between all consecutive 180° pulses.

After each CPMG pulse sequence, a wait time T_(w) of several seconds isnecessary in order to allow the hydrogen to re-polarize to anequilibrium condition. Therefore, although the actual pulse sequence maylast only for milliseconds, each measurement takes several seconds, oncethe wait time is taken into account. It is therefore advantageous toissue a CPMG sequence only during time windows when lateral tool motionis slow to improve the chances for a valid NMR measurement and not towaste valuable measurement time. Since the exact delay between CPMGpulse sequences is not critical, is it possible, to extend, ifnecessary, the minimum required delay T_(w) by additional time in orderto start a new measurement in a favorable time window.

Conversely, the thin shell of the sensitive volume 36 (FIG. 6A),suggests that moderate lateral tool motion can be helpful byautomatically selecting a new sensitive volume within a small delayafter a CPMG measurement. By monitoring the lateral tool motion, thetool can determine when such a condition exists and can speed up themeasurement cycle by shortening the wait time T_(w).

(1) Signal Processing Using Phase-Alternated Pairs

In accordance with the present invention the echo responses acquiredwithin a CPMG sequence are preferably digitized and recorded in twochannels: in-phase and in-quadrature. In a first processing step, datafrom consecutive CPMG sequences are added echo-by-echo andchannel-by-channel to form a phase-alternated pair (PAP). The first datahalf of a PAP comes from a CPMG sequence as shown in FIG. 33(a), thesecond half is supplied by a CPMG sequence as shown in FIG. 33(b). Thedifference between the sequences shown in FIGS. 33(a) and 33(b) is thatin the latter the phases of all 180° pulses have been inverted. The neteffect of using PAP in accordance with the present invention is thatartifacts, which tend to be coherent with the phases of the 180° pulses,tend to cancel out, while the NMR signal, whose phase is tied to theinitial 90° pulse, is amplified. For those skilled in the art it shouldbe obvious that the same effect can be achieved by inverting the phaseof the 90° pulse from 90° to 270°, accompanied by subtracting—instead ofadding—the resultant echo streams.

In order to achieve the beneficial effect of PAP accumulation, bothparts of a PAP must be valid NMR measurements. An important part of thepresent invention is that information about the tool motion isincorporated into the NMR sequencing. Thus, in a specific embodiment, ifduring any CPMG sequence the lateral motion exceeded the allowablelimits, the resultant data set is discarded and the computer/signalprocessor 30 attempts to re-acquire a valid data set. To this end, thecomputer delays by the appropriate wait time T_(w) and repeats theprevious CPMG sequence. In accordance with this embodiment, only whenthe data has been validated the sequence proceeds to the nextmeasurement.

To describe motion management as used in the present invention in theproper context, it should be noted that it is well known in the art thatthe amplitude of the first echo is fairly good approximation of thenumber of hydrogen atoms in the fluid state and therefore can becalibrated to read fluid-filled porosity. Therefore, in accordance witha first embodiment of this invention, the simplest possible CPMGmeasurement consists of only one 90° pulse, one 180° pulse and onesignal acquisition. The total time from the first pulse to the end ofthe signal acquisition in this case is about 0.5 msec. It has beendemonstrated by numerical modeling and by experiments that a lateraldisplacement of about 0.1 mm is allowable within this time periodwithout causing an undesired change in echo amplitude. This translatesinto a maximum limit for transversal velocity of 0.2 m/sec.

Alternatively, the CPMG pulse sequence may be continued with more 180°pulses and data acquisitions. Subsequent echoes may become more and moredepressed because the sensitive volume shifts laterally through theformation. However, as shown in FIGS. 35a and 35 b, even in this case anapproximation of the initial echo amplitude can be recovered. FIG. 35ashows typical echo amplitudes without motion present. The peaks of theechoes are curve-fitted and extrapolated back to the time of the initial90° pulse, as known in the art. The resultant amplitude A_(o) is adirect measure of formation porosity. With tool motion present, the echoamplitudes become more and more depressed as shown in FIG. 35b, andcurve-fitting and back-extrapolation yields a possibly differentapproximate value for the initial amplitude A_(o). The followingdescription illustrates approaches to motion management used inaccordance with the present invention to correct NMR measurement signalsobtained from moving tools.

(2) Motion Management

Referring back to FIG. 1, in a specific embodiment, the motionmanagement aspect of this invention comprises accelerometers 24 alongwith a signal processor 30 for processing the accelerometer signals andthe NMR signals from the formation, and a conventional data transmitter150 to transmit the data to the surface. In certain embodiments, anadditional high-pass filter 44 can also be used. Alternatively, anotheradvantageous embodiment shown in FIG. 29 comprises acoustic sensors 26and an inclinometer 48. Yet another embodiment of the motion managementused in accordance with the present invention, shown in FIG. 30,comprises acoustic sensors 26 and a magnetometer 50. Anotheradvantageous embodiment shown in FIG. 31 comprises contact sensors 46and an inclinometer 48. Still another advantageous embodiment shown inFIG. 32 comprises contact sensors 46 and a magnetometer 50. In theembodiments of FIGS. 29 through 32, the signal processor 30 is forprocessing, in addition to the NMR signals from the formation, theacoustic sensor, contact sensor, inclinometer, and magnetometer signals,as the case may be. The various embodiments having accelerometers,acoustic sensors, contact sensors, inclinometer, and/or a magnetometercan be used in accordance with the present invention to manage thelateral motion of the tool 40 in the borehole 36, i.e., to account forit as part of the signal processing algorithm.

Tool Motion Regimes

In the preferred embodiment, the type of NMR measurement performed isadapted to the tool motion regime in an automatic fashion. The followingmotion regimes can be identified and correspond to particular NMRmeasurements:

1. Stationary tool. If no lateral motion is detected, the tool canreplicate NMR measurements similar to wireline operation. Because themeasurement volume does not substantially change during the course of afew 100 milliseconds, the CPMG sequence is extended to include typically501 pulses and 500 echoes. The wait time between experiments is chosento allow the same sensitive volume to re-polarize between CPMG sequencesand is on the order of 5 seconds.

2. Normal drilling mode. Some lateral motion is present, but the speeddoes not exceed 0.2 m/s. This means that at least the first echo isalways valid and possibly more echoes, depending on the instantaneousvelocity. In this motion regime, in a preferred embodiment the toolselects a shortened CPMG sequence of typically 51 pulses and 50 echoes.The wait time is shortened by about a factor of one-half to 2.5 seconds.Data taken during an interval where the velocity exceeds 0.2 m/s arediscarded and the sequencer attempts to re-acquire the same data. Alldata are time-stamped and stored in non-volatile memory, together withmotion data immediately acquired before and after the NMR measurement.The echo amplitudes are curve-fitted as shown in FIG. 35, and theobtained amplitude A_(o) is converted into porosity units and reportedin real-time to the surface by means of the mud-pulse system.

3. Whirling mode. In this mode, the lateral velocities are typicallyoutside acceptable limits. In a specific embodiment the tool selects theshortest possible CPMG sequence consisting of only two pulses and asingle echo. The processor waits for the motion detection to identify amoment when the instantaneous speed is below 0.15 m/s and attempts anNMR measurement that lasts 0.5 msec. Immediately after the NMR dataacquisition, the instantaneous velocity is checked again. If it does notexceed 0.2 m/s, the NMR data point is accepted, otherwise rejected. Theminimum delay between measurements is about 2.5 sec, but the actualdelay can be highly variable, depending on the frequency of motionwindows.

4. Stick/slip mode. In this mode, certain time windows exist in whichthe tool is almost stationary. The tool monitors the motion data todetermine whether or not these windows are longer than a 50-echo CPMGsequence. If not, the tool falls back to the single-echo of a whirlingcondition (3). If yes, the CPMG sequence compatible with normal drillingis selected (50 echoes). The tool triggers the CPMG measurement at theonset of a “sticking” condition, but no sooner than about 2.5 secondsafter the last measurement. Motion data taken during and after the CPMGsequence is recorded to determine if the window was wide enough tovalidate all 50 echoes.

In accordance with the present invention the tool is programmable beforebeing deployed in the borehole. The operator can choose which of theoperating modes (1)-(4) can be selected during run time. Also, certainmodes may be forced, bypassing the downhole decision logic.

FIG. 9 is a block-diagram of the motion management algorithm used inaccordance with the present invention. The algorithm starts at 200 andproceeds to measure the tool's lateral acceleration and rotation speed(RPM) at 205. If in decision block 210 the RPM measurement indicatesstick-slip motion (235), the next step 240 is to forecast the stickphase and then calculate the average and maximum lateral velocities inblock 245. Next the computed quantities are compared at 250 to somepredetermined constants. In particular, if average lateral velocity isless than or equal to V_(a1) and the maximum lateral velocity is lessthan or equal to V_(a2), then it is safe to process the NMR signals, andthe algorithm proceeds to prepare NMR sampling at 260.

If the conditions in decision block 250 are not met, the velocitiesindicate that the tool is undergoing whirling motion possibly combinedwith stick-slip motion, and the whirling portion of the algorithm mustbe followed.

If the RPM measurement does not indicate stick-slip motion at 210, thenthe next step 215 is to calculate the average and maximum lateralvelocities. If in decision block 220 the average lateral velocity isshown to be less than or equal to a predetermined value V_(a1) and themaximum lateral velocity is also shown less than or equal to anothervalue V_(a2), then the velocities indicate normal drilling motion 225.In such case, it is safe to proceed with the NMR sampling in block 230.However, if the average lateral velocity is greater than V_(a1) or themaximum lateral velocity is greater than V_(a2), then the velocitiesindicate that the tool is undergoing whirling motion, indicated in block265. In that case, further calculations are needed.

When the velocities indicate whirling motion, the next step 270 is tocalculate the duration of time for which the lateral velocity is lessthan V_(a2). If that duration is greater than or equal to a parameterT_(a), then the next step 275 is to calculate the duration in which thelateral velocity is less than a parameter V_(a1). If this secondduration is greater than or equal to T_(a), then the NMR measurement 280should be made when the lateral velocity is less than or equal to V_(a1)and the absolute value of the lateral acceleration is less than apre-determined value. If this second duration is less than T_(a), thenthe NMR measurement 285 should be made when the lateral velocity is lessthan or equal to V_(a2) and the absolute value of the lateralacceleration is less than a pre-determined value. Similarly, if theduration of time for which the lateral velocity is less than V_(a2) isalso less than T_(a), then the second duration calculation is not neededand the NMR measurement 285 should be made when the lateral velocity isless than or equal to V_(a2) and the absolute value of the lateralacceleration is less than a pre-determined value.

In the preferred embodiment, the maximum motion criterion in the MWDmode is that the lateral velocity must be less than or equal to about0.2 m/s. However, to be conservative, the criterion maybe set at 0.15m/s to provide a 33% margin of safety. The variables used in theembodiment illustrated in FIG. 9 are thus as follows:

V_(a1)=0.10 m/s

V_(a2)=0.15 m/s

T_(a)=10 ms

In the sliding/wiping mode, the maximum motion criterion may bedifferent due to different NMR sampling times, and the cut-off frequencyfor the high-pass filter 44 (see FIG. 1) may be different depending onthe drill collar rotation speed.

Accelerometer-based Motion Management

In a preferred embodiment, four accelerometers 24 are placed around thecircumference of the drill collar 10 as shown in FIG. 8. Accelerometersac1 and ac2 are at opposite ends of one diameter, and accelerometers ac3and ac4 are at opposite ends of a perpendicular diameter. The in-planecomponent of the gravitational acceleration g sin α, is oriented in thevertical plane, and the lateral acceleration vector is in some arbitrarydirection. In typical applications, the drill collar rotates at angularvelocity w, producing centripetal acceleration ac=rw². As discussedabove, the motion of the drill collar 10 for an arbitrary inclinedorientation (as shown in FIG. 7) is governed by Eqs. [5]. To eliminatethe gravity terms in Eqs. [5], a high-pass filter 44 (see FIG. 1) may beused. The frequency cut-off for the high-pass filter is above that ofthe typical drill string rotation speeds. The governing equations thenbecome simplified to those of Eqs: [1], so that the drill collarrotation speed S and the magnitude and direction of the lateralacceleration ax may be determined by Eqs. [2], [3], and [4].

Because the tool 40 is typically rotating, the initial measurement ofthe lateral acceleration ax is with respect to a rotating coordinatesystem (x′,y′), as shown in FIG. 39. However, the velocity anddisplacement of the tool 40 are needed in the fixed reference frame ofthe borehole (earth). Thus, the lateral acceleration measurement must beconverted to a fixed coordinate system (x,y) by way of a coordinatetransformation. Referring to FIG. 39, the orthogonal components of thelateral acceleration ax in the rotating coordinate system arerepresented as A_(x)′ and A_(y)′. The rotating coordinate system isrotationally displaced from the fixed coordinate system by an angle φ.Therefore, the orthogonal components, A_(x) and A_(y), of the lateralacceleration ax in the fixed coordinate system are calculated asfollows:

A _(x) =A _(x)′ cos φ+A _(y)′ sin φ  Eq. [7]

A _(y) =−A _(x)′ sin φ+A_(y)′ cos φ  Eq. [8]

The angle φ (in radians) is obtained from the following relation:

φ=φ_(o) +ωt  Eq. [9]

where φ_(o) is the initial value of the angle φ, ω is the angularvelocity (in radians/second) of the drill collar 10, and t is time (inseconds). Because the actual value of the initial condition, φ_(o), isnot important for purposes of this invention, φ_(o) may be assumed to bezero.

After converting the lateral acceleration ax into the fixed referenceframe, the signal processor 30 is then used to integrate the lateralacceleration ax once to obtain the tool's lateral velocity, and twice toobtain the tool's lateral displacement. The signal processor 30 thenuses these parameters in conjunction with the motion managementalgorithm as shown in FIG. 9 to predict acceptable time windows in whichto make NMR measurements. After NMR measurements are made, the signalprocessor 30 also verifies that they were made within acceptable levelsof lateral motion.

Acoustic-Sensor-Based Motion Management

In the acoustic sensor versions of the apparatus (FIGS. 29 and 30), aplurality of at least two acoustic sensors 26 are placed around theperimeter of the drill collar 10. Similarly; in the contact sensorembodiments of the apparatus (FIGS. 31 and 32), a plurality of at leasttwo contact sensors 46 are placed around the perimeter of the drillcollar 10. Instead of measuring the tool's lateral acceleration, theseembodiments measure the tool's lateral displacement by sensing thetool's distance from the borehole wall at a plurality of times. Then,the signal processor 30 differentiates the displacement measurementsonce to obtain the tool's lateral velocity, and twice to obtain thetool's lateral acceleration.

In the single-echo acquisition modes of whirling and stick/slip, themeasurement can tolerate lateral velocities of up to about 0.5 m/s.Although up to 50% signal amplitude is lost at the upper limit of therange 0.2-0.5 m/s, this loss can be recovered by estimating the averagelateral velocity during the NMR measurement (0.5 msec). In FIG. 14, theshaded area represents the portion (overlap) of the sensitive volumethat is the same at the time of the 90° pulse and the first-echo signalacquisition window. The percentage that the shaded area bears to theoriginal, full cross-sectional area of the sensitive volume is the sameas the percentage that the received NMR signal amplitude bears to theNMR signal amplitude that would have been received if the tool hadremained stationary. Thus, the plot of FIG. 17 shows the relationshipbetween the fraction of the original sensitive volume at the time of theNMR echo and lateral tool movement between the 90° pulse and the NMRecho. The signal processor uses this relationship to apply theappropriate correction factor to the NMR signal.

FIG. 15 shows how to calculate the area of the overlapping portions ofthe measurement volume of FIG. 14. Referring to FIG. 15, because radiir_(o) and r_(i) are much greater than the displacement x, the angles αand β are approximately equivalent. Therefore, for a single quadrant,the area enclosed by circle #1 is given by the relation

A ₁=(¼)(πr _(o) ²)[(π/2−α)/π/2]  Eq. [10]

and the area enclosed by circle #2 is given by the relation

 A ₂=(¼)(πr _(i) ²)[(90+α)/90]  Eq. [11]

The area of the triangle is given by the relation

A _(t)≈(½)(x)[(r _(o) +r _(i))/2]  Eq. [12]

Thus, the area A_(a) of the hatched portion is given by the equation

A _(a) ≈A ₁−(A ₂ +A _(t))  Eq. [13]

FIG. 16 illustrates the computation in another embodiment in which thearea A_(b) of the hatched portion 16 is determined by the equation

A _(b) =A ₂ −A ₁  Eq. [14]

FIG. 17 is a plot of A_(a) expressed as a fraction of the initial areaof the sensitive volume for x<1.5 mm. This plot is for a preferredembodiment of the NMR MWD tool for which the sensitive volume has anominal diameter of 13.5 inches and a thickness of 1.5 mm.

If the sampling rates of the acoustic sensors 26 or contact sensors 46,as the case may be, do not exceed the rotation speed of the tool 40 by asufficient margin, then the rotation of the tool 40 is no longernegligible with respect to the lateral displacement. In such cases, atleast three acoustic sensors 26 or contact sensors 46 must be used inconjunction with either an inclinometer 48 or magnetometer 50 (FIGS. 29to 32). Referring to FIG. 37, the acoustic sensors 26 or contact sensors46 are represented by the symbols S₁, S₂, and S₃, which are preferablyspaced uniformly (i.e., 120 degrees apart) around the perimeter of thetool 40, as shown. The inclinometer 48 or magnetometer 50 is representedby the symbol S₄ (hereafter referred to as instrument S₄). Forconvenience, the location of instrument S₄ is taken to be oppositesensor S₁. The sensors S₁, S₂, and S₃ yield displacement measurementsd₁, d₂, and d₃, which provide an estimate of the diameter of theborehole 36. As the tool 40 rotates, the instrument S₄ produces asinusoidal electrical signal, E, as a function of the rotationalposition, f, of the tool 40 within the borehole 36 (FIGS. 37 and 38).Together with the slope, dE/dφ, of the curve in FIG. 38, the value of Edetermines the angle φ. The coordinate system (x,y) in FIG. 33 is fixedto the borehole 36, but the actual orientation of this coordinate systemwithin the borehole is arbitrary. For convenience, the coordinate system(x,y) has been chosen such that the peak electrical signal, E, occurswhen instrument S₄ is in the negative y direction (i.e., φ=π/2), and theangle φ is referenced from the x axis according to the right hand rule.The values of d₁, d₂, d₃, and φ determine the location of the center ofthe tool 40 within the borehole 36, which may be described in eitherrectangular coordinates (x,y) or radial coordinates (r,θ), as shown inFIG. 33. By measuring the location (x₀,y₀) and (x₁,y₁) of the center ofthe tool 40 at two successive times t₀ and t₁, the lateral displacement,d, of the tool 40 may be calculated as follows:

d=[(x ₁ −x ₀)²+(y ₁ −y ₀)²]^(1/2)  Eq.[15]

The lateral displacement, d, may then be used to correct the NMR signal,as discussed above. Alternatively, the components of the lateralvelocity (V_(x) and V_(y)) the lateral velocity (V_(L)), and therotation speed (RPM) of the tool 40 may be calculated by the followingequations

V _(x)=(x ₁ −x ₀)/(t ₁ −t ₀)  Eq. [16]

V _(y)=(y ₁ −y ₀)/(t ₁ −t ₀)  Eq. [17]

V _(L)=[(V _(x))²+(V _(y))²]^(1/2)  Eq. [18]

RPM=(60/2π)*(φ₁−φ₀)/(t ₁ −t ₀)  Eq. [19]

for use in the motion management algorithm of FIG. 9.

Persons skilled in the art will recognize that a potential difficultyexists for either an inclinometer 48 or magnetometer 50 in certainorientations of the tool 40. Specifically, if instrument S₄ is aninclinometer, the electrical signal, E, will be constant rather thansinusoidal if the inclinometer is in a vertical orientation. Similarly,if instrument S₄ is a magnetometer, the electrical signal, E, will beconstant rather than sinusoidal if the magnetometer is aligned with theearth s magnetic field. In either case, the consequence of a constantelectrical signal, E, is that the angle φ is not determinable, whichprecludes the calculation of the lateral displacement and rotation speedof the tool 40. This potential difficulty may be eliminated by includingboth an inclinometer 48 and a magnetometer 50 in the tool 40 so that,for any orientation of the tool 40 within the earth, at least one of thetwo instruments will produce a sinusoidal electrical signal, E.

NMR Signal Processing Using Motion Management

The preferred method for collecting NMR data during motion is shown as aflow chart in FIG. 36. It is assumed that the principal motion regimehas been determined according to the algorithm illustrated in FIG. 9. Inaccordance with the present invention at the start 400 of the algorithmthis information allows to optimally select the length of the CPMG pulsetrain and the wait time between CPMG pulse/echo measurements in block405. As a safety measure, prior before pulsing the transmitter, thesystem gain is determined at 410 by injecting a reference signal intoauxiliary coil (See FIG. 5). If the received signal returned through themain antenna 14 is too low, the tool shuts down at 445 for apredetermined time, e.g., 15 minutes. Otherwise, the actual CPMGsequence is delayed at 415 until a suitable time window with aninstantaneous lateral velocity of less than 0.15 m/s occurs. In thestationary and normal drilling modes, this condition should always bemet.

Next, a CPMG pulse-echo sequence of pre-determined length is broadcastand echo data are acquired at 420, as known in the art. After every CPMGsequence, the lateral velocity is re-checked at 425. If it exceeds 0.2m/s, the newly acquired data is discarded at 455 and the tool delays bya pre-determined time T_(ww) at 450. Typically, T_(ww) is shorter thanT_(w) to take advantage of the fact that the tool moved to a freshvolume. After T_(ww) seconds, the search for a suitable time windowresumes at 415 as shown in FIG. 36.

If the check at 425 indicates valid data, the data set is accepted at430; the tool delays for the pre-determined time T_(w) are set at 435,and the cycle repeats with the acquisition of the second half of a PAP.Once the PAP set is complete, at 440 the algorithm exits.

As noted above, the beneficial effect of phase-alternated signalaveraging is achieved in accordance with the present invention bycoherent accumulation of NMR data, coupled with the progressivesuppression of non-NMR artifacts. Most of the latter is comprised ofpulse-induced magneto-acoustic ringing from metallic and magneticstructures within the tool. It has been determined by experimentationthat the patterns of said artifacts tend to change cyclically with thetool's orientation and bending.

With reference back to FIG. 32, in a preferred embodiment the signalprocessor 30 derives an estimate of the tool's orientation from the setof motion sensors, in particular from magnetic pick-up of the earth'smagnetic field and/or from the gravity component of the accelerometerdata. Within the time windows imposed by the lateral motion, processor30 attempts to synchronize the NMR measurement with the rotation of thetool string. Under normal drilling conditions, lateral velocities arealmost always within acceptable limits and rotational synchronizationtends to dominate the time sequencing of the NMR data collection. Underwhirling conditions, the opportunities for synchronizing the NMRmeasurement to the tool orientation start to disappear. If the timewindows derived from lateral velocities drop under a pre-determinedvalue, processor 30 no longer attempts synchronization based on toolorientation.

While the foregoing has described and illustrated aspects of variousembodiments of the present invention, those skilled in the art willrecognize that alternative components and techniques, and/orcombinations and permutations of the described components andtechniques, can be substituted for, or added to, the embodimentsdescribed herein, It is intended, therefore, that the present inventionnot be defined by the specific embodiments described herein, but ratherby the appended claims, which are intended to be construed in accordancewith the following well-settled principles of claim construction: (a)Each claim should be given its broadest reasonable interpretationconsistent with the specification; (b) Limitations should not be readfrom the specification or drawings into the claims (e.g., if the claimcalls for “antenna”, and the specification and drawings show a coil, theclaim term “antenna” should not be limited to a coil, but rather shouldbe construed to cover any type of antenna); (c) The words “comprising”,“including”, and “having” are always open-ended, irrespective of whetherthey appear as the primary transitional phrase of a claim or as atransitional phrase within an element or subelement of the claim; (d)The indefinite articles “a” or “an” mean one or more; where, instead, apurely singular meaning is intended, a phrase such as “one”, “only one”,or “a single”, will appear; (e) Words in a claim should be given theirplain, ordinary, and generic meaning, unless it is readily apparent fromthe specification that an unusual meaning was intended; (f) an absenceof the specific words “means for” connotes applicants' intent not toinvoke 35 U.S.C. § 112 (6) in construing the limitation; (g) Where thephrase “means for” precedes a data processing or manipulation“function,” it is intended that the resulting means-plus-functionelement be construed to cover any, and all, computer implementation(s)of the recited “function”; (h) a claim that contains more than onecomputer-implemented means-plus-function element should not be construedto require that each means-plus-function element must be a structurallydistinct entity (such as a particular piece of hardware or block ofcode); rather, such claim should be construed merely to require that theoverall combination of hardware/firmware/software which implements theinvention must, as a whole, implement at least the function(s) calledfor by the claim's means-plus-function element(s); (i) ameans-plus-function element should be construed to require only the“function” specifically articulated in the claim, and not in a way thatrequires additional “functions” which may be described in thespecification or performed in the preferred embodiment(s); (j) Theexistence of method claims that parallel a set of means-plus-functionapparatus claims does not mean, or suggest, that the method claimsshould be construed under 35 U.S.C. § 12 (6).

What is claimed is:
 1. A nuclear magnetic resonance (NMR) logging toolfor conducting measurements of a down hole formation, comprising: (a) apermanent magnet having longitudinal axis; (b) a nonmagnetic metal drillcollar surrounding the permanent magnet; (c) an antenna mounted on theoutside of said drill collar; and (d) one or more soft-magnetic elementsinstalled in proximate relationship with the antenna, said soft-magneticelements shaping radio frequency (RF) fields generated by the antenna;(e) a motion detector generating signals corresponding to motions of theNMR logging tool.
 2. The NMR tool of claim 1 further comprising a downhole signal processor for processing NMR signals from said formation. 3.The NMR tool of claim 1 further comprising a drill bit for drilling aborehole in said down hole formation.
 4. The NMR tool of claim 1 furthercomprising one or more pre-polarization magnets positioned proximatesaid permanent magnet along its longitudinal axis.
 5. The NMR tool ofclaim 4 comprising two pre-polarization magnets each prepolarizationmagnet positioned at an end of said permanent magnet.
 6. The NMR tool ofclaim 1, wherein said permanent magnet comprises a plurality of magnetsegments.
 7. The NMR tool of claim 6, wherein said plurality of magnetsegments is made of rare earth materials.
 8. The NMR tool of claim 1further comprising one or more auxiliary antennas to enable sampling ofradio frequency (RF) flux from the antenna.